Field of the Invention
The invention relates to a catalytic converter and a method for purifying the exhaust gas from a combustion engine which is operated with excess air, in particular from a motor vehicle engine.
It is known that a large number of different groups of pollutants are formed during the combustion of fuels, e.g. diesel fuel. To eliminate or reduce the levels of these groups of pollutants, different catalytic conversions and different catalytic converters or active materials are required. To reduce the levels of emissions in the exhaust gas, there are generally a plurality of cleaning systems, each with a specific action, connected in series or in parallel. For example, different catalytic systems are used to reduce NOx compounds from those used for the oxidative conversion of hydrocarbon compounds.
A method for the selective catalytic reduction of NOx in oxygen-containing exhaust gas is described, for example, in European patent EP 0 487 886 B1. The exhaust-gas flow is mixed with a nitrogen-containing reducing agent by means of a metering device. Then, this mixture is brought into contact with an SCR catalytic converter which uses the selective catalytic reduction (SCR) method to break down nitrogen oxides contained in the exhaust gas, using the reducing agent, to form molecular nitrogen and water. Highly volatile or gaseous organic compounds (HVOC and VOC) and compounds with a moderate to low volatility which accumulate on particles (SOF) are only insufficiently influenced by the SCR catalytic converter in the low-temperature range. To reduce the levels of these emissions, a further catalytic converter system in the form of an oxidation catalytic converter is required. Therefore, an oxidation catalytic converter as described, for example, in EP 0 692 301 A2 is generally connected downstream of an SCR catalytic converter. Therefore, in conventional catalytic converter systems for reducing the levels of NOx and hydrocarbon emissions, the complexity of the equipment and the associated costs are relatively high. Furthermore, in some instances the exhaust-gas flow is subjected to still further cleaning steps, which, however, further increases the complexity of the equipment and the costs.
It is accordingly an object of the invention to provide a simplified catalytic converter system for purifying exhaust emitted from a combustion engine operated with excess air, that overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type, and in particular allows the concentration of both volatile organic compounds and nitrogen oxides in the exhaust gas, thereby taking up a small installation space and achieving low production costs.
With the foregoing and other objects in view there is provided, according to the invention, a system for purifying the exhaust gas from a combustion engine operated with excess air, comprising an exhaust line transporting exhaust gas from said combustion engine, a feeding device for bringing into contact with said exhaust gas at least one nitrogenous reducing agent and a downstream catalytic device for diminishing the concentration of nitrogen oxides and volatile organic compounds in said exhaust gas including a novel catalytically active material comprising the oxides TiO2, V2O5, CaO and SiO2 and at least one oxide selected from the group consisting of WO3 and MoO3, wherein the proportions of said oxides are as follows (% by weight):
When in contact with the catalytically active material according to the invention, the reducing agent introduced from the feeding device is catalytically decomposed, to afford ammonia, and the nitrogen oxides contained in the exhaust gas stream are converted owing to the SCR activity of the catalyst, by reaction with the aid of the ammonia produced, into harmless nitrogen and water.
In accordance with another feature of this invention, there is also provided a method for purifying an exhaust gas from a combustion engine operated with excess air and containing nitrogen oxides, in which the exhaust gas from the combustion engine is transported through an exhaust line into contact with at least one reducing agent supplied from a feeding device and thence into contact with a catalytically active material comprising the oxides TiO2, V2O5, CaO and SiO2 and at least one oxide selected from the group consisting of WO3 and MoO3, wherein the proportions of said oxides are as follows (% by weight):
The catalytic converter according to the invention can include known auxiliary and filler materials. Such an auxiliary or filler is understood as meaning the fillers, auxiliaries and binders which are customary for ceramic materials, such as for example ceramic fibers or glass fibers as filler/supporting agents, resins, shellac or cellulose as pore-forming agent, and butyl glycol, isopropanol, ethyl glycol or polyethylene oxide as film-forming auxiliary.
Surprisingly, it has been found that with the active material according to the invention the levels of both volatile organic compounds and nitrogen oxidesxe2x80x94the latter in the presence of a nitrogen-containing reducing agentxe2x80x94in an exhaust gas can be reduced particularly effectively. In the low-temperature range, the organic compounds accumulate on the active material by chemisorption; at relatively high temperatures they are primarily oxidized to form carbon dioxide and water. At the same time, however, the active material also catalyzes the reaction between the NOx compounds and the reducing agent which is fed to the exhaust gas.
The catalytic converter and the method therefore allow simultaneous conversion of organic compounds and NOx using the same active material. Where previously two separate catalytic bodies, each in their own housing or at least housing compartment were required for said conversions, now only a single-piece catalytic converter is required. This reduces the installation space required, which is advantageous in particular in motor vehicles, and the production costs.
Particularly good reduction levels are achieved if the active material is composed of, in % by weight, 70-95% TiO2, 2-10% WO3 and/or MoO3, 0.1-5% V2O5, 0.1-8% CaO and 0.1-10% SiO2.
Furthermore, it has been found that the CO oxidation activity of the active material can be improved by doping with or admixing with elements selected from the group consisting of Pt, Pd, Rh, Ru and Ir, without the capacity for simultaneous reduction of the levels of NOx and hydrocarbons being significantly impaired. Said elements selected from the group of platinum metals in the periodic system of the elements (PSE) may be added individually or as a mixture and in any desired inorganic form or compound, for example as halides, nitrates, or oxides.
A particularly good CO oxidation activity is attained if said platinum metals are added in an amount of from 0.01-5% by weight.
To produce the catalytic converter, the catalytically active material described above together with fillers and auxiliaries when present can be extruded to form a solid extrudate. A monolithic catalytic body of this type has, for example, a large number of parallel flow passages, through which the exhaust gas can flow, passing through it. Another possibility consists in coating an inert catalytic body, for example made of ceramic or metal, with the active material. In each case, the active material is prepared by mixing, milling and kneading of the oxides or their precursor compounds, if appropriate with the addition of customary ceramic auxiliaries and fillers. A catalytic body made from an active material of this type or a metallic or ceramic support body which is coated with an active material of this type, for example in honeycomb or plate form, is dried at temperatures of between 20xc2x0 C. and 100xc2x0 C. and calcined at a temperature of from 200xc2x0 C. to 800xc2x0 C.

The BET surface area of the catalytic converter should be in the range from 30 m2/g to 150 m2/g. The pore volume, measured using the Hg penetration method, should be in the range from 100 mm3/g to 1000 mm3/g with a monomodal or polymodal pore radius distribution.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a catalytic converter and method for cleaning exhaust gas, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.